In the manufacture of liquid crystal cells, two glass plates are joined together with a layer of a liquid crystal material sandwiched between them. The glass substrates have conductive films thereon (at least one of which is transparent, such as an ITO film) that can be connected to a source of power to change the orientation of the liquid crystal material. Various areas of the liquid crystal cell can be accessed by proper patterning of the conductive films. More recently, thin film transistors (hereinafter TFT) have been used to separately address areas of the liquid crystal cell at fast rates. Such liquid crystal cells are useful for active matrix displays such as TV and computer monitors.
As the requirements for resolution of liquid crystal monitors has increased, it has become desirable to separately address a plurality of areas of the liquid crystal cell, called pixels. Since about one million pixels are present in modern displays, at least the same number of transistors must be formed on the glass plates so that each pixel can be separately addressed.
Different types of thin film transistors are in current use; two common types are the inverted amorphous silicon TFT and a polysilicon TFT. The amorphous silicon TFT requires deposition of a gate dielectric layer over a patterned gate material with an amorphous silicon layer thereover. The gate dielectric layer can be a single layer of one material, such as silicon nitride, or it can be made of two layers of different materials. In the case when two layers are employed, the bottom layer is typically a metal oxide or silicon oxide, and the top layer, adjacent to the amorphous silicon layer, is typically silicon nitride. Metal contacts are deposited over the amorphous silicon film, which can have a thin layer of doped silicon thereover to improve contact between the amorphous silicon and the overlying aluminum contacts.
The polysilicon TFT has a layer of polysilicon as the active semiconductor material, which is deposited over a thick silicon oxide underlayer. On top of the polysilicon there is a thin layer of silicon oxide for the gate dielectric, and another layer for the gate contact. Source and drain contacts are formed in contact with the polysilicon layer.
The silicon oxide film must be of high quality when it is used as the gate dielectric, either in the inverted amorphous silicon TFT or the polysilicon TFT. The silicon oxide must be conformal and free of pinholes or voids in order to prevent leakage current between the gate contact and the active semiconductor material. In addition, the silicon oxide must have a high resistance to electrical breakdown, must be electrically stable and must have a minimum amount of fixed charge or charge trapping sites in order for the transistor electrical characteristics to be stable and reliable.
Silicon oxide is deposited also directly on a bare glass substrate as an underlayer to act as a barrier between the glass substrate and the transistor for inverted amorphous silicon TFTs, polysilicon TFTs and top-gate amorphous silicon TFTs. In this application, the requirements for film quality of silicon oxide are similar to the requirements for silicon oxide used as a gate dielectric, and there is an additional requirement that the film act as a barrier to prevent the diffusion of chemical contaminants from the glass substrate to the transistor.
An additional application for silicon oxide is as an electrically insulating layer in polysilicon TFTs. The requirements for this application are similar to the requirements for silicon oxide used as a gate dielectric layer, though perhaps less stringent.
Because of the large size and weight of glass substrates, which can be for example about 350.times.450.times.1.1 mm in size, generally large reaction chambers are required for deposition of thin films thereon, and large and often slow transfer equipment is needed to transfer the substrates from one reaction chamber to another for sequential deposition of the thin films for thin film transistor manufacture.
However, recently a vacuum system has been developed having multiple chambers that can bring a plurality of substrates to vacuum, heat them batch wise to CVD temperatures, transfer them singly to specially designed CVD or other processing chambers that can deposit thin films of, inter alia, silicon oxide, and transfer them back to a cooling chamber, all without leaving a vacuum environment. However, in order to maximize the efficiency of such a system, the processing time for each substrate must be kept in a range of about 30 to 120 seconds. Thus thick silicon oxide films must be deposited at high deposition rates, while thin silicon oxide films can be deposited at lower deposition rates.
Thus it would be highly desirable to be able to deposit conformal films of electrically stable silicon oxide having a low charge by chemical vapor deposition over a range of deposition rates.